W1,W2 background is over-subtracted around many systems

[moustakas]

Issue reported by Si-Yue Yu:

I am recently using the SGA images to understand the galaxy structure. However, I found that the WISE images 
are a bit abnormal. For example, the attached NGC0895_W1.png is the W1 image of NGC0895 
([NGC0895-image-W1.fits.fz](https://portal.nersc.gov/project/cosmo/data/sga/2020/data/035/NGC0895/NGC0895-image-W1.fits.fz)). 
There are some black (negative value) regions around the galaxy. It looks like the background is somehow 
over-subtracted. The attached NGC0895_W1_model.png is its W1 model image 
([NGC0895-largegalaxy-model-W1.fits.fz](https://portal.nersc.gov/project/cosmo/data/sga/2020/data/035/NGC0895/NGC0895-largegalaxy-model-W1.fits.fz)). ' 
Instead, there are many small black holes around the galaxy.

[moustakas]

I also independently found this issue in my own work, and it looks like turning off UNWISE_MODEL_SKY_DIR fixes the problem in many cases (probably dependent on galaxy diameter). E.g., here's the data mosaic for NGC3162 with UNWISE_MODEL_SKY_DIR off (left) and on (right), so things look much better.